1. Field of the Invention
The invention resides in the automotive art. More specifically, the invention relates to an anti-theft system for a motor vehicle, in which doors are locked or unlocked, or an electronic immobilizer is released, only after proof of authorization is provided.
2. Description of the Related Art
A prior art anti-theft system--described in German patent disclosure DE 38 02 248 A1--has an antenna device in the driver's door. If a user wants to enter the vehicle, then a question-response dialog is triggered by actuating a trip switch. An interrogation signal is sent from the antenna device in the vehicle to a transponder carried by the user. The transponder transmits an enciphered response signal back if it receives the interrogation signal. In the motor vehicle, the response signal is compared with an expected desired signal, and if the two agree (successful authentication), then the doors are locked or unlocked.
The antenna device is implemented in the form of two frame antennas that are perpendicular to one another. The antennas generate electromagnetic fields. These fields induce a voltage in a transponder coil of the transponder. For the induced voltage to be as high as possible, the field lines must penetrate the transponder coil to a sufficient extent. This occurs whenever the field lines of the magnetic field generated extend not in a single plane but rather in at least two planes. The two frame antennas are disposed at right angles to one another for this reason. However, this requires a very large amount of installation space.
It can happen, however, that the portable transponder may accidentally be positioned with the winding face of its transponder coil such that the winding face still extends parallel to the field lines of the magnetic field. The transponder coil is then penetrated inadequately if at all by the magnetic field, so that the interrogation signal is not received by the transponder, or it is received with insufficient amplitude intensity.
In another prior anti-theft system--described in copending application No. 08/749,199 and German patent disclosure DE 195 42 441 A1--an antenna is used that comprises two frame antennas disposed close together and in the same plane. To generate a three-dimensional magnetic field, the two frame antennas are controlled separately from one another but in phase offset from one another. This creates a magnetic field that is moved back and forth in three dimensions. Once again, it can still happen that the winding faces of the transponder coil will not be sufficiently penetrated by magnetic field lines of the magnetic field generated by the two frame antennas.
Furthermore, the two frame antennas cannot always be disposed close together. This is the case for instance if one frame antenna is disposed in the front door and the other in the rear door. In that case, no--or an only slightly pronounced--magnetic field that moves back and forth is created, since the range of the magnetic field is limited.